(1) Field of the Invention
The present invention relates to a semiconductor memory device, more particularly to a read-only memory (ROM) device in which information can be written through a mask, i.e., a mask ROM.
(2) Description of the Prior Art
To improve the data processing ability of semiconductor integrated circuits (IC's), it is important to increase the degree of integration of the ROM's used therein. With ROM's of a binary logic structure, however, the limit has substantially been reached in miniaturization of the composite transistors. To further increase the density of ROM's, therefore, use is now being made of so-called multivalue-logic ROM's, i.e., ROM's of a three or more value logic structure.
A multivalue-logic ROM is formed by transistors having three or more different threshold voltages. In the case of a three-value logic ROM, the transistors having a high threshold voltage, a medium threshold voltage, and a low threshold voltage. Due to the restrictions imposed by the source voltage, however, it is not possible to have a large difference between the threshold voltages of a ROM. This necessitates severe process control when forming the ROM. There are inevitable variations between the threshold voltages of different ROM's due to variations in the channel length, thickness of the gate oxidation film, dose of impurities into the channel region, heat treatment conditions, depth of the source and drain regions, and the like. This results in a loss of reliability of fixed data and a lower production yield of semiconductor IC's provided with such ROM's.